Method of producing high strength thin steel



United States Patent 3,139,359 METHOD OF PRODUCING HIGH STRENGTH THINSTEEL Eric R. Morgan, Pittsburgh, Pa., assignor to Jones &

Laughlin Steel Corporation, Pittsburgh, Pa., a corporation ofPennsylvania N0 Drawing. Filed June 12, 1961, Ser. No. 116,300 7 Claims.(Cl. 148--12.1)

This invention relates to a method of manufacturing thin steel of highstrength and good ductility. It is more particularly concerned with amethod of making such steel in which the steel is nitrided either beforeor after it is reduced to gauge.

Inexpensive thin steel sheet and strip of high strength and goodductility is a desirable article of commerce, particularly as a base fortin plate. Thin steel in the form of wire also has many uses. Forreasons of cost steel strip to be tinned is conventionally made of lowcarbon steel containing no deliberately added alloying elements. Thestrip is hot rolled to intermediate gauge and is brought to the desiredgauge by cold rolling. Those skilled in the art are aware that thedegree of reduction of the steel obtained in cold rolling has a greatinfluence on the physical properties of the product, and adjust therelative reductions eifected by hot and cold rolling accordingly.

The bulk of the tin plate used in tin cans has thicknesses on the orderof .010". The cold rolling necessary to reduce the steel to that gaugealso reduces its ductility undesirably and it is therefore conventionalin the production of steel of that type to anneal the cold reducedmaterial to soften it. The annealing, of course, reduces the tensile andyield strengths of the material. Conventional tin plate displays tensilestrengths of about 45,000 to 50,000 p.s.i. and elongations of about 20to 25% in two-inch gauge length. Those properties are adequate forconventional tin cans.

Can manufacturers would use even thinner tin plate if its tensilestrength were sufficiently higher than conventional material tocompensate for the thinner walls of cans made therefrom. Thinner tinplate Weighs less and costs less than conventional tin plate per unit ofarea. Modern rolling mills can roll strip to the thinner gauges desiredbut the strip after annealing displays very nearly the same unit tensilestrength as conventional material and so does not produce cans of therequired strength.

It is an object of my invention to provide a method of manufacturingsteel strip or sheet of light gauge and of unit tensile strength andyield strength substantially higher than that of conventional material.It is another object to provide a method of manufacturing such steelhaving unimpaired deformability. Other objects of my invention willappear from the description thereof which follows.

I have found that if nitrogen-containing carbon steel in the form ofstrip, sheet or wire is cold reduced to an intermediate gauge, and isthen annealed and further cold reduced to the desired gauge, theresulting product possesses high tensile strength and yield strength andsubtantial deformability. I have found that if the nitrided steel iscooled after annealing so as to retain its nitrogen in solution, itstensile strength is further increased by 3,139,359 Patented June 30,1964 strain-aging. I have also found that the steel can be nitridedwhile it is being annealed.

The desired improvement in tensile strength of the steel is inverselyproportional to its thickness. If tin plate .007" thick, for example, isto be manufactured into cans having the same strength as those made fromconventional tin plate .010" thick, the tensile strength of the thinnersteel must be about 65,000 to 70,000 p.s.i. My invention makes itpossible to obtain that improvement in tensile strength without decreasein deformability of the material, and is capable of producing thinmaterial having even higher tensile strengths. My invention also makesit possible to provide thin steel of conventional gauge but withimproved tensile and yield strength.

In all cases the steel to which I refer herein is plain carbon steel ofthe composition range shown in Table I.

In one embodiment of my process presently preferred by me the steel isworked by hot rolling followed by cold rolling to an intermediate gauge.The material is then annealed in coil form in a non-oxidizing atmosphereto which is added about 25% by volume of anhydrous ammonia. In orderthat the nitriding of the steel be accomplished uniformly the wraps ofthe coiled strip must be disposed so that the nitriding atmosphere hasaccess to the entire surface of the coil. In this condition the coil canbe simultaneously annealed and nitrided quite rapidly, the time requireddepending on the temperature to which the steel is heated. I find thetemperature range from about 1000 F. to 1200 F. to be satisfactory, thecorresponding times ranging from about one-half hour to two hours. Underthose conditions the nitrogen content of the steel ranged from about.010% to .030%. The annealed and nitrided strip was cooled to roomtemperature and brought to the desired final gauge by a further coldreduction of about 2 /2 Those skilled in the art will understand thatthe treatment above described can be given the steel in the form ofsheets if the sheets are disposed during annealing and nitriding (sothat the furnace atmosphere has access to the entire surface thereof.

Steel strip rolled to nominal .0061" thickness treated as abovedescribed exhibits tensile strength of about 76,000 lbs. p.s.i. andductility, as measured by elongation over a two-inch gauge length,between 6 and 12%. The tensile strength of the material provided by myprocess is somewhat better than that of conventional material of .0118"nominal thickness made from rephosphorized steel. The ductility of theconventional material, measured as elongation over a two-inch gaugelength, was about 18%.

When the material produced by my process was aged by dipping it inmolten tin at a temperature of 650 F. for five seconds, its tensilestrength was increased to about 80,000 lbs. p.s.i.

- The process of my invention may also be practiced in another preferredembodiment by continuously annealing and nitriding steel strip or Wirein the form of strand. The time during which the steel is at annealingand nitriding temperature is much shorter in continuous strand annealingthan in coil annealing so that the temperature of the strand must beincreased somewhat. Because of the short time for nitriding I accomplishthis step in an atmosphere of undiluted anhydrous ammonia. Where thesteel is held at maximum temperature for at least 30 seconds, a treatingtemperature of about 1100 F. to about 1300" F. is satisfactory andraises the nitrogen content of the steel to .07% and upwards.

The strand was raised to temperature in times ranging from to 36seconds, held at temperature for seconds, and then cooled. Steel in theform of strand can be cooled rapidly, or quenched, which is not possiblefor steel in coil form. I find that it is desirable to cool the strandto a temperature of about 200 F. or so in about 30 seconds. Steelprocessed as above described exhibits tensile strengths on the order of68,000 to 70,000 p.s.i. and elongations between 6 and 19% over atwo-inch gauge length. The same material treated in the same way but notnitrided exhibits tensile strength on the order of 58,000 lbs. p.s.i.When the nitrided steel is given a further reduction of 2 /2 to 3% andaged one hour at the temperature of boiling water its tensile strengthis increased another 3,000 to 4,000 p.s.i. Without significantimpairment of its ductility.

It is desirable to have the nitrogen dissolved in the steel rather thanprecipitated as nitrides. Quenching the steel after annealing andnitriding holds the nitrogen in solution, but aging the steel causesnitrogen to precipitate. Steel wires nitrided and annealed as abovedescribed and then quenched in water were found to contain .030%nitrogen in solution, while wires of the same steel treated in the sameway but allowed to age at 200 C. for a week were found to contain only.005 to .008% nitrogen in solution. The wires before aging displayedtensile strengths of 45,000 to 48,000 p.s.i., while after aging theirtensile strengths dropped to 40,000 to 41,000 p.s.i. If, however, thequenched material is strain-agedthat is, given a light reduction andthen aged, its tensile strength is further increased, as has beenmentioned, and its nitrogen remains largely in solution.

It is convenient to nitride the steel at the same time it is beingannealed and this procedure makes possible the attainment of much highernitrogen contents in the steel than can be readily obtained by addingnitrogen to the steel when it is melted. However, steel which isnitrogenized during its melting is satisfactory for my process as longas its nitrogen is not combined with aluminum or like elements known inthe art which prevent it from going into solution in the steel. Idesignate nitrogen in steel suitable for my process as uncombinednitrogen.

The final cold Working of my steel includes reductions up to about 15%.I find that value to be critical as far as the deformability of thesteel is concerned. I measure deformability by bending a coupon of thematerial, cut transverse to the rolling direction, 180 around a diameterequal to its original gauge. Table II identifies the effect of variousdegrees of final reduction of steel of my invention and shows that aboveabout 15% reduction the number of bends which the steel withstands ishalved. The steel of Table II had a composition within the limits ofTable I and contained .0l0%.012% uncombined nitrogen. It was hot andcold reduced to .0195" thickness, and annealed in coils. It is apparentfrom Table II that thin steel of adequate deformability having tensilestrength well over 70,000 p.s.i. is readily attainable by the method ofmy invention.

4 TABLE 11 Physical Properties as Function of Cold Reduction TransverseDirection Yield Tensile Percent Percent Reduction Strength, Strength,Elonga- Bend p.s.i. p.s.i. tion 2 in. Test It is also apparent fromTable II that my method requires a minimum of about .010% uncombinednitrogen in the steel. While smaller amounts will produce somestrengthening effect, that effect will not be sufiicient within thecritical limit of final cold reduction to increase the tensile strengthin inverse proportion to the reduced thickness of the steel. Highernitrogen contents make it possible to obtain the desired strengtheningwith smaller amounts of final cold reduction.

The reduction effected by cold rolling must be more than about 3% if theyield strength of the material is to be raised. This again may be seenfrom the data of Table II. The minimum reduction varies slightly withthe composition of the steel and its thickness, but it must be more thana conventional temper reduction to bring about a useful increase inyield strength of the material.

I claim:

1. The method of producing thin carbon steel of high yield and tensilestrength and substantial deformability comprising annealing cold reducedsteel of intermediate gauge containing up to about .14% carbon and atleast .010% uncombined nitrogen and then cold reducing the annealedintermediate gauge steel in amount more than about 3% but not more thanabout 15% to the desired gauge.

2. The method of producing thin carbon steel of high yield and tensilestrength and substantial deformability comprising annealing cold reducedsteel of intermediate gauge containing up to about .14% carbon and atleast about .010% uncombined nitrogen so that the nitrogen is held insolution in the steel and then cold reducing the annealed intermediategauge steel in amount more than about 3% but not more than about 15% tothe desired gauge.

3. The method of producing thin carbon steel of high tensile strengthand substantial deformability comprising cold reducing carbon steelcontaining up to about .14% carbon to an intermediate gauge, annealingthe cold reduced steel in a nitriding atmosphere so as to nitride thesteel to at least about .010% uncombined nitrogen content, and then coldreducing the annealed intermediate gauge steel in amount not more thanabout 15% to the desired gauge.

4. The method of claim 3 in which the steel is annealed and nitrided incoil form at a temperature between about 1000 F. and 1200" F. in anon-oxidizing atmosphere containing anhydrous ammonia.

5. The method of claim 3 in which the steel is continuously annealed andnitrided in the form of strand at a temperature between about 1100 F.and 1300 F.

6. The method of claim 3 in which the steel is quenched after annealingand before cold reducing to the desired gauge.

7. The method of claim 5 in which the strand is held at temperature inan atmosphere of anhydrous ammonia for at least about 30 seconds, andthen cooled.

6 References Cited in the file of this patent UNITED STATES PATENTS2,361,434 Surtees Oct. 31, 1944 FOREIGN PATENTS 618,645 Great BritainFeb. 24, 1949 OTHER REFERENCES Samans: Engineering Metals and TheirAlloys, pp. 371-374 and 741-742, 1949.

Metals Handbook, 1948 edition, published by the A.S.M., pages 354-355and 439 relied upon.

3. THE METHOD OF PRODUCING THIN CARBON STEEL OF HIGH TENSILE STRENGHTAND SUBSTANTIAL DEFORMABILITY COMPRISING COLD REDUCING CARBON STEELCONTAINING UP TO ABOUT .14% CARBON TO AN INTERMEDIATE GAUGE, ANNEALINGTHE COLD REDUCED STEEL IN A NITRIDING ATMSOPHERE SO AS TO NITRIDE THESTEEL TO AT LEAST ABOUT .010% UNCOMBINED NITROGEN CONTENT, AND THEN COLDREDUCING THE ANNEALED INTERMEDIATE GAUGE STEEL IN AMOUNT NOT MORE THANABOUT 15% TO THE DESIRED GAUGE.